(1) Field of the Invention
The invention relates to a method of fabricating semiconductor memory structures, and more particularly, to the formation of a floating gate with a sloping sidewall for a Flash Memory Cell.
(2) Description of the Prior Art
Flash EEPROMs are a well-known class of semiconductor devices in the art. These devices are used in many digital circuit applications where binary data must be retained even if the application system power is removed. Further, these devices allow the data to be altered, or re-written, during normal operations.
EEPROM memory devices employ floating gates; that is Field Effect Transistor (FET) gates completely surrounded by an isolating layer such as silicon oxide. The presence of charge on these floating gates effectively shifts the threshold voltage of the FET. This effect can be detected by additional circuitry such that the charge state of the floating gate can be used to represent binary information. Specifically, FLASH EEPROM memories employ EEPROM cells in a configuration that allows for the bulk erasing, or flashing, of large blocks of memory cells in a normal circuit application without using any external data erasing source, such as ultra-violet light.
FIG. 1 shows a cross sectional view of a partially completed prior art EEPROM memory cell. The cell contains a substrate 11 typically composed of lightly P- doped monocrystalline silicon. Isolation regions 12 extend above and below the substrate surface to effectively isolate this memory cell from surrounding cells. The region defined along the substrate surface between the two isolation regions 12 is called the active region. A tunneling oxide layer 13 overlays the substrate 11 and the isolation regions 12. A polysilicon floating gate 14 overlays the tunneling oxide 13. The tunneling oxide 13 serves as an isolator between the floating gate 14 and the substrate 11. An interpoly dielectric film 15, typically comprised of oxide-nitride-oxide, or ONO, overlays the floating gate 14. Another layer of polysilicon forms the control gate 16 of the memory cell. The interpoly dielectric film 15 serves as an isolator between the control gate 16 and the floating gate 14. The overlaying layers of control gate 16, interpoly dielectric 15, floating gate 14, and tunneling oxide 13 over substrate 11 form a stacking gate structure.
Data is stored in the EEPROM cells by the storage of a charge on the floating gate 14. Because this gate 14 is electrically isolated from both the substrate 11 and the control gate 16, a charge can be stored for indefinite periods without any voltage applied to the gate 14. To charge or write data to the floating gate 14, a voltage must be applied from the control gate 16 to the substrate 11. This voltage is divided across the capacitor formed by the control gate 16, the interpoly dielectric 15, and the floating gate 14, and the capacitor formed by the floating gate 14, the tunneling oxide 13, and the substrate 11. If the voltage from the floating gate 14 to the substrate 11 is large enough, charge movement will occur as electrons tunnel from the substrate 11 to the floating gate 14 through the tunneling oxide layer 13. When the voltage from control gate 16 to substrate 11 is reduced or removed, the charge is trapped on the floating gate 14 and the data is retained in the memory cell. The presence of this charge increases the threshold voltage of the memory cell FET, and this can be detected by a cell sense circuit.
A prominent feature of the prior art cell shown in FIG. 1 is severe topology introduced by the field oxide 12 isolation. Because the polysilicon floating gate 14 overlaps this isolation 12, as well as the tunneling oxide layer 13, all of the subsequent layers of material reflect this topology. Notably, a sharp corner exists at the polysilicon floating gate edge 18. This sharp corner causes problems with the integrity of the interpoly dielectric 15. The effective breakdown probability of the interpoly dielectric is increased, as is the amount of charge leakage between the floating gate 14 and the control gate 16. The effect of this charge leakage is the performance of the slash memory cell is deteriorated. As charge leaks off the floating gate 14, the effective threshold voltage of the memory cell FET is altered, compromising the data held on the cell.
A prior art attempt to reduce the sharpness of the floating gate edge is taught in U.S. Pat. No. 5,635,416 to Chen et al. Chen et al teaches the formation of tunnel oxide and the first part of the floating gate polysilicon prior to the formation of the isolation oxide. The second part of the floating gate extends on to the isolation oxide. U.S. Pat. No. 5,573,979 to Tsu et al shows a sloped polysilicon sidewall for a memory device. However, Tsu is directed at capacitor electrodes and teaches a sloping sidewall method for what are termed unreactive and barrier layers such as platinum and titanium nitride, respectively, in the formation of three-dimensional capacitor nodes for DRAM. U.S. Pat. No. 5,554,564 to Nishioka et al shows a method of rounding the bottom capacitor electrode through the oxidation of the noble metals used.